1. Field of the Invention
The invention relates to an image intensifier tube, comprising a conversion screen with a substrate on which there is provided a seed layer which is separated by an intermediate layer from a scintillation layer for converting incident radiation of a first wavelength into radiation of a second wavelength. Such an image intensifier tube is used inter alia in an X-ray examination apparatus in order to convert an X-ray image into a light image and to increase the brightness thereof.
2. Description of the Related Arts
An image intensifier tube of this kind is known from Japanese Patent Application JP 62-245471 (publication No. 64-89131).
The known image intensifier tube comprises an entrance section with a conversion screen which comprises a substrate, for example in the form of an aluminium foil. On the substrate there is provided a seed layer which consists of crystalline particles of an akalihalide material, for example caesium iodide (CsI), having a thickness of 15 .mu.m or less. On the seed layer there is provided a thin intermediate layer of a metal or metal oxide, preferably aluminium, which has a thickness of between 10 nm and 300 nm, preferably approximately 100 nm, and which follows the shape of the crystalline particles of the seed layer. On the intermediate layer there is vapour deposited a scintillation layer which has a thickness of from approximately 250 to 450 .mu.m and consists of columnar crystals of a fluorescent alkalihalide, such as sodium-doped caesium iodide (CsI:Na). The crystalline particles of the seed layer, covered by the aluminium of the intermediate layer, act as nuclei for the formation of the scintillation layer with columnar crystals. These columnar crystals provide a light guiding effect for the light of the second wavelength which is produced by absorption of incident radiation of the first wavelength in the scintillation layer.
The intermediate layer of the known image intensifier tube is formed by vapour deposition of a metal or a metal oxide in an inert gas atmosphere, for example a xenon atmosphere. Such a vapour deposition method produces an intermediate layer of a powdery material. The intermediate layer in the known image intensifier tube is constructed as a layer which consists of one or more metals or metal oxides and is conceived so that the intermediate layer absorbs radiation of the second wavelength, notably light, produced in the conversion screen. Consequently, a part of the light produced in the conversion screen is lost to the formation of the electron image by the photocathode and the sensitivity of the known X-ray image intensifier tube for the conversion of incident radiation is degraded.